Passing an object from QML to C++ - c++

Question

I am working on a simple app to demonstrate integration of C++ with QML but I have problems. In nutshell I want to create a C++ object in qml on fly and pass it to C++ for processing. Here is my code.
import QtQuick 2.4
import QtQuick.Window 2.2
import test 1.0
Window {
visible: true
MainForm {
anchors.fill: parent
mouseArea.onClicked: {
console.log("clicked")
var student = Qt.createComponent("Student")
student.name = "Hello frome QML";
console.log( student.name ) // good
school.addStudent( student )
}
}
}
Student.h
#ifndef STUDENT_H
#define STUDENT_H
#include <QObject>
class Student : public QObject
{
Q_OBJECT
QString m_name;
public:
explicit Student(QObject *parent = 0);
~Student();
Q_PROPERTY(QString name READ name WRITE setName NOTIFY nameChanged)
QString name() const
{
return m_name;
}
signals:
void nameChanged(QString arg);
public slots:
void setName(QString arg)
{
if (m_name == arg)
return;
m_name = arg;
emit nameChanged(arg);
}
};
#endif // STUDENT_H
Now when I addStudent() to the school class, there is where there is trouble. The concern .cpp file below.
#include "school.h"
#include <QDebug>
School::School(QObject *parent) : QObject(parent)
{
}
School::~School()
{
}
// why does this not work? it cause compiler error..can't access private members of QObject
//void School::addStudent(Student student)
//{
//// students.append( &student );
//// qDebug() << "added";
//// qDebug() << student.name();// << " was added to school";
//}
void School::addStudent(Student * student)
{
students.append( student );
qDebug() << "added";
qDebug() << student->name();// if this line is there, it breaks the application
}
The questions are also in comments inside the code but the summaries:
Why does addStudent() crashes when I try to access student.name? I can access it though in the qml file after I set it.
Why does my project not compile if I pass Studentby value like seen in the code?
Additional thoughts
It seems like it may have to do with the C++ object getting destroyed by the time C++ function execute. What is the life cycle of C++ object created in QML? When does it gets destroyed? Could the lifetime of the object be an issue here?

Answer 1

Assume that you register the Student type to QML via qmlRegisterType in cpp file.
In your QML, Qt.createComponent("Student") does not create a Student object but a QQmlComponent. Try to set properties other than name and it still works since the object is not a Student. Instead, you should create an object from a pre-defined component via Component.createObject:
MainForm {
id: mainForm
mouseArea.onClicked: {
var student = studentComponent.createObject(mainForm);
student.name = "Hello frome QML";
school.addStudent( student );
}
Component {
id: studentComponent
Student{}
}
}
And everything works fine now.
Back to your code,
//QML
var student = Qt.createComponent("Student")
student.name = "Hello frome QML";
console.log( student.name ) // good
No, it's not good. student.abcdefg = "Hello from QML" works, too. Add console.log(student) and you can see that student is not a Student.
Why does addStudent() crashes when I try to access student.name? I can access it though in the qml file after I set it.
It crashed since the object passed from QML is not a pointer Student. QML engine passes a null pointer to this function. What you accessed in QML is not a Student.
Why does my project not compile if I pass Student by value like seen in the code?
Because Student is an QObject, which is not copyable. Pass by pointer instead.
What is the life cycle of C++ object created in QML? When does it gets destroyed? Could the lifetime of the object be an issue here?
The lifetime is not a issue in your QML since what you created is a component, not an object. When creating Student object in this answer,
var student = studentComponent.createObject(mainForm);
the newly created object sets mainForm as it's parent. The object won't be deleted until mainForm is released. Except you delete it explicitly.

Answer 2

Try moving the declaration of the function QString name() const in the declaration of class Student to before the Q_PROPERTY macro. Or specify "public:" again before the name function. The macro is resetting the access specifier to "private".